This invention relates to a fluidized bed combustion system and a method of operating same and, more particularly, to such a system and method in which a recycle heat exchanger is formed integrally with the furnace section of the system.
Fluidized bed combustion systems are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. Water is passed in a heat exchange relationship to the fluidized bed to generate steam. The combustion system includes a separator which separates the entrained particulate solids from the gases from the fluidized bed in the furnace section and recycles them back into the bed. This results in an attractive combination of high combustion efficiency, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed utilized in the furnace section of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well defined, or discrete, upper surface. Other types of fluidized beds utilize a "circulating" fluidized bed. According to this technique, the fluidized bed density may be below that of a typical bubbling fluidized bed, the air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith.
Also, circulating fluidized beds are characterized by relatively high solids recycling which makes it insensitive to fuel heat release patterns, thus minimizing temperature variations, and therefore, stabilizing the emissions at a low level. The high solids recycling improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle, and the resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption.
However, several problems do exist in connection with these types of fluidized beds, and more particularly, those of the circulating type. For example, a sealing device such as a seal pot, a syphon seal, or an "L" valve and a hot expansion joint are required between the low pressure cyclone discharge and the higher pressure furnace section of the system, and the transfer of the separated particulate material from the cyclone back to the fluidized bed has to be done by a gravity chute or a pneumatic transport system. The addition of these components add to the cost and complexity of the system. Also in these types of systems the particulate material recycled from the cyclone to the fluidized bed has to be at a fairly precise temperature. This requires an increased furnace height or the installation of wear-prone surfaces in the upper furnace to cool the particulate material before being reinjected into the fluidized bed to the appropriate temperature. Alternatively, heat may be removed from the recycled solids by submerging cooling surfaces in a second fluidized bed situated between the collector outlet and the return point to the main fluidized bed. A unit of this type, termed a recycle heat exchanger, has the advantage of very high heat transfer rates. As with any fluid bed heat exchanger, the simplest method of controlling the amount of heat transfer accomplished would be by varying the level of solids in the recycle heat exchanger.
Situations exist in which a sufficient degree of freedom in choosing the recycle bed height is not available, such as for example, when a minimum fluidized bed solids depth or pressure is required for reasons unrelated to heat transfer. In this case, the heat transfer may be controlled by diverting a portion of the particulate material so it does not contact and become cooled by the recycle heat exchanger. When the correct portions of cooled and uncooled material are subsequently recombined, the desired final material temperature may be obtained. For example, different type valves, such as "plug valves" and "L valves," have been used to bleed a portion of the solids passing through the bed and/or to directly control the flow rates from both a heat exchanger bed and a material-pressure control bed. However, these type arrangements require the use of moving parts within the solids system and/or need solids flow conduits with associated aeration equipment.